This invention relates to soldering machines and methods and more particularly to an improved drag soldering machine and method for the soldering of through-hole printed wiring boards (PWB).
Soldering is widely employed for the fabrication of many different products, especially products in the electronics field and notably through-hole printed wiring boards (PWB). Typically, electronic components with extending leads are connected to circuitry on the PWBs by soldering the leads into preexisting holes on the PWB. For the mating of electronic components to the PWBs a variety of different types of soldering techniques may be employed. The most common technique is wave soldering which involves moving a PWB in one direction across the crest of a stationary, continuously replenished wave of molten solder. The solder wave contacts the bottom of the PWB and simultaneously wets the holes in the PWB and the electronic component leads extending through these holes. Upon moving past the solder wave, excess solder drains away and a quality solder joint remains between the leads and the PWB. Another technique, drag soldering, involves lowering a pallet supporting the through-hole PWB with electronic components in place on the board into a solder bath until the PWB contacts with the solder. The PWB is then dragged a predetermined distance along the surface of the bath, after which it is lifted from the bath. Just as with wave soldering the solder simultaneously wets the holes in the PWB and the electronic leads of the components. After the PWB is lifted from the solder bath, excess solder drains and solder joints remain which attach the component leads to the PWB.
Prior to the introduction and promotion of the wave soldering technique, drag soldering was a popular means for soldering PWBs. Wave soldering has became a more popular soldering technique because in most instances the wave soldering technique minimizes defects such as bridging and excess solder on the bottom side of the PWB. However, recent modifications in PWB design and the addition of heat sinks have caused wave soldering techniques to be inadequate for heat transfer for these PWBs.
With technological advances the thickness of the PWB has become an important design factor because as new condensed electrical components become available the need to develop more densely populated PWBs emerges. This dense population of circuitry causes higher operating temperatures of the PWB whicn in turn demands the addition of heat sinks to the PWB design. Furthermore, this increased circuit density requires multilayer PWBs to provide enough circuit paths for the components to properly communicate with other components on the PWB. This results in a thicker PWB having a greater thermal mass, thereby exceeding the capability of the wave soldering technique to uniformly heat the PWB.
Using a wave soldering machine on thick PWBs is inadequate because of nonuniform heating across the length of the board caused by the wave soldering technique. Specifically, while the cold leading edge of the PWB requires a greater heat transfer rate from the solder wave to raise the leading edge to a minimum temperature, this same heat transfer rate causes the trailing edge of the PWB to overheat. The high rate of heat transfer causes the portion of the PWB approaching the solder wave to accumulate heat through heat conduction before it actually contacts the solder wave. Consequently, as the PWB passes over the solder wave the PWB portion which has already been substantially heated from previous heat conduction now passes over the solder wave and receives additional and excessive heating from the solder wave. Overheating the PWB during the solder process is unacceptable and subsequently wave soldering of thick PWBs is not acceptable. This is particularly a problem with thick PWBs and printed wiring assemblies with heat sinks since longer contact periods with the solder wave permit more extensive heat transfer.
On the other hand, the drag soldering technique permits the entire length of the PWB to dwell in the heated solder for a fixed period. This permits a relatively uniform heating of the PWB across its entire length, which is especially advantageous to the soldering of thick PWBs. Furthermore, a drag soldering machine generally is much less complex than a wave soldering machine and consequently maintenance costs are much less. Ideally for the soldering of electronic components to a thick PWB, the uniform heating of the drag soldering technique is desirable but the problem of defects caused by bridging and excess solder must be solved.
An object of this invention is to provide a solution to the current problem of drag solder defects caused by bridging and excess solder on the PWB.
FIG. 1 illustrates a schematic of a drag soldering machine. A PWB 2 with electronic components 4 in place enters the drag solder machine along a linear entry guide 6 and is moved along a drag guide 8 over the surface 10 of a molten solder bath 12. The PWB 2 is then removed from the solder bath 12 along a linear exit guide 14. The PWB 2 is moved in and out of the solder bath 12 using a transport means 16, which may consist of a motor-driven chain (not shown) that pulls a pallet (not shown) into which the PWB 2 would be placed. Note that in actuality the PWB is placed in a pallet and it is the pallet that is guided. For simplicity the PWB will be exactly the length of the pallet such that when the pallet enters and leaves the solder bath, the PWB will do so simultaneously. For this reason the PWB will be discussed without reference to the pallet. In actuality the pallet will probably be longer than the PWB.
In drag soldering a significant factor effecting the quantity of solder remaining on the solder joints and the PWB and a significant factor also effecting bridging, in which solder bridges between two terminals, is the vertical velocity of the PWB as it separates from the solder in the solder bath. This vertical velocity is also known as the peel-out velocity of the PWB with respect to the solder bath since this motion causes the PWB to separate, or peel, from the solder bath. A high vertical velocity leaves a large quantity of solder on the PWB, while a low velocity leaves a lesser quantity of solder on the PWB. A high vertical velocity also creates more bridging. Control of this influential vertical velocity has not been accomplished on existing drag soldering machines.
A more detailed sketch of a typical drag soldering machine exit guide is shown in FIG. 2. Typically, the linear exit guide 14 consists of a linear ramp oriented at an angle of approximately 13 degrees from the horizontal plane created by the solder bath surface 10 (see FIG. 1). This feature is highlighted in FIG. 2, which shows the PWB 2 in three positions. Position 18 shows the PWB on the drag guide 8. Position 20 shows the PWB 2 in transition between the drag guide 8 and the linear exit guide 14. Position 22 shows the PWB 2 on the linear exit guide 14. The vertical velocity of the PWB 2 as it leaves the solder bath depends on the velocity at which the PWB 2 moves across the drag guide 8 onto the linear exit guide 14 and the contour of the linear exit guide 14. Ideally, the vertical velocity at the point of separation of the PWB 2 from the solder of the solder bath should be constant across the length of the PWB 2. Since the actual point of separation of the PWB from the solder bath is strictly a function of the distance between the PWB and the solder bath and typically this distance is about 1/8", the critical vertical velocity is that velocity occurring at the portion of the PWB that is 1/8" from the solder bath. Note that this separation distance of about 1/8" is a function of the solder and the temperature of the solder. For simplicity and as an approximation, the vertical velocity of the PWB at the instant the PWB leaves the solder bath, not when the PWB is a 1/8" distance from the solder bath, will be used in this discussion.
Defining a leading edge 24, a middle point 26 and a trailing edge 28 on the PWB 2, it can be shown that the vertical velocity of the PWB from the solder bath for the configuration shown in FIG. 2, which is typical of existing drag soldering machines, is not constant. Once the PWB 2 passes a transition point 30 between the drag guide 8 and the linear exit guide 14, separation between the PWB 2 and the solder bath begins. Given a velocity V of the PWB 2 parallel to the linear exit guide 14, when the leading edge 24 of the PWB 2 passes the point 30 of transition, the vertical velocity increases from zero to V sin .theta., where .theta. in this instance is equal to 13.degree., while the vertical velocity of the trailing edge 28 remains at zero. From the time the leading edge 24 passes the transition point 30, until the trailing edge 28 passes the transition point 30, the vertical velocity of any intermediate point, including the midpoint 26, may range in speed between zero and V.times.sin .theta.. The result of this is that the vertical velocity at the point of separation of the PWB 2 from the solder bath changes across the length of the PWB. The velocity of those points on the PWB closest to the leading edge 24 is much higher than that of those points closer to the trailing edge 28. Relating this to the drag soldering process, a larger quantity of solder is left deposited on those points of the PWB closer to the leading edge 24 than on those points closer to the trailing edge 26 since as mentioned earlier a high speed of withdrawal leaves a large quantity of solder on the PWB. This larger quantity of solder is the major cause of bridging on the PWB.
FIG. 3 is a graph which illustrates the fraction of the PWB which separates from the solder bath as the PWB is moved from the drag guide to the linear exit guide. FIG. 3 actually shows the location of the point of separation of the solder from the surface of the PWB. As mentioned earlier, this occurs when the PWB is about 1/8" above the solder bath. As the PWB begins to travel along the linear exit guide the distance between the board and the solder bath increases until the leading edge of the board is a vertical distance of about 1/8" from the solder bath. At this point the leading edge of the PWB breaks away from the solder. As the board continues to travel from the drag guide to the linear exit guide, the portion of the PWB that becomes separated from the solder bath increases rapidly. FIG. 3 illustrates that when the PWB has travelled about 20% of its length, 80% of the PWB has separated from the solder bath. Similarly, the vertical velocity of the point of separation in this region is very high which causes excessive amounts of solder to be deposited across the leading section the PWB. Furthermore, only 20% of the PWB is drained during the remaining 80% of the PWB travel onto the exit guide which necessarily indicates a relatively slow vertical velocity at the point of separation for 80% of the PWB. Ideally the PWB should be withdrawn from the solder bath so that a uniform vertical velocity exists across the length of the PWB at the point of separation between the PWB and the solder bath.
It is an object of this invention to provide a means by which a printed wiring board may be separated from a solder bath such that a uniform vertical velocity exists across the distance of the PWB at the point of separation between the PWB and the solder bath resulting in approximately equal amounts of solder deposited across the length of the PWB.